One technique of using a charged particle beam to transfer a pattern to a substrate is known as “divided-reticle pattern transfer.” Divided-reticle pattern transfer involves dividing a pattern into individual exposure units, termed “subfields,” that are defined on a “divided” or “segmented” reticle and exposed in a prescribed order subfield-by-subfield. As the pattern is transferred from the segmented reticle to the substrate, the subfield images are positioned, or “stitched” together, on the substrate so that they collectively form a single contiguous transferred pattern.
Divided-reticle microlithography systems typically utilize “stencil” reticles, comprising a reticle membrane defining numerous openings (or “apertures”) that allow transmission of the beam through a charged-particle-beam (CPB) optical system to the substrate. These apertures are shaped according to respective individual pattern elements to be transferred. During transfer-exposure, the stencil reticle is positioned on a reticle stage within the CPB-optical system such that the charged particle beam (or “illumination beam”) passes through a subfield of the divided reticle and becomes “patterned” by the distribution and configuration of pattern elements in the illuminated subfield. This “patterned beam” then passes through projection lenses that collectively focus the patterned beam on a corresponding “transfer subfield” on the substrate, which is mounted on a substrate stage located downstream from the reticle stage. After a subfield is exposed, the charged particle beam may be deflected, or the reticle and substrate stages repositioned, to illuminate and transfer the next subfield. The CPB-optical system located upstream of the reticle and used for irradiating the subfields on the reticle is termed the “illumination-optical system,” whereas the CPB-optical system located between the reticle and the substrate is termed the “projection-optical system.”
As the patterned beam is transmitted through the projection-optical system, some degree of image blur occurs. The degree of blur is influenced by the individual characteristics of the subfield being transferred and by the resolving power of the projection-optical system. The following have been identified as causes of image blur:
(1) geometrical aberration inherent in the projection-optical system itself, or caused by, for example, mechanical irregularities arising during assembly of various components of the projection-optical system and thermal deformations of the projection-optical system arising during operation thereof;
(2) chromatic aberration resulting from energy distributions generated as the CPB passes through the reticle;
(3) image-placement errors during image formation resulting from external forces such as the vibration of the apparatus during exposure; and
(4) fluctuations in the image-formation position resulting from the space-charge effect.
Geometrical aberration can be reduced by incorporating various aberration-correcting optics into the projection-optical system. The aberration-correcting optics can be used for reducing mechanical irregularities and for providing effective temperature control for reducing thermal deformation of projection-optical system.
Chromatic aberration is insignificant when stencil-type reticles are used, and thus can be largely ignored for purposes of the present invention.
Image-placement errors during image formation can be reduced by measuring the relative positions of the substrate stage, the reticle stage, and the projection-optical system using an interferometer. The interferometer is connected to a controller of the microlithography apparatus (“exposure apparatus”) so that adjustments to the projection-optical system may be made in real time.
Geometrical aberration, chromatic aberration, and apparatus vibration are all causes of image blur directly attributable to the exposure apparatus. Variations in image-formation position caused by the space-charge effect, however, depend from the individual characteristics of the subfield being transferred. Specifically, the distribution of pattern elements in a subfield affects the current of the illumination beam passing through the subfield, thus altering the distribution of space-charge in the patterned beam and changing the influence of the space-charge effect. To date, there have not been effective countermeasures to the variation in space-charge effects caused by varying pattern-element densities during transfer-exposure of multiple patterns using CPB microlithography.